Infos from : http://europa.eu/scadplus/constitution/minister_en.htm

Creating the post of Minister for Foreign Affairs was one of the main innovations of the Constitution. The purpose of introducing such a role was to make the European Union's external action more effective and coherent, the Minister for Foreign Affairs becoming in effect the voice of the Union's  common foreign and security policy  (CFSP).
The creation of the post of Minister for Foreign Affairs was discussed at length both by the  members of the Convention  and by the  Inter-Governmental Conference  (IGC)
This institutional innovation is the result of merging the functions of the High Representative for the CFSP and the External Relations Commissioner. Provision is made for the creation of the post of Foreign Affairs Minister in Article I-28 of the Constitutional Treaty. The Minister's role consists in conducting the CFSP and the European security and defence policy (ESDP), using all the instruments at his or her disposal.
The Foreign Affairs Minister will not, however, have sole charge of the Union's external representation. The Constitution specifically assigns the task of representing the Union to the Commission, except for CFSP issues. Article I-22 of the Constitution provides that the  President of the European Council  , as well as preparing and chairing the work of the European Councils, is responsible at his or her level for ensuring the external representation of the Union on issues concerning its common foreign and security policy, without prejudice to the responsibilities of the Minister for Foreign Affairs. The Constitution does not, however, go into detail about how these functions are to be divided between the European Council President and the Foreign Affairs Minister, leaving their respective roles for institutional practice to decide.